The thesis investigates the development of decision competence in probabilistic decision environments. Six experiments investigate probabilistic judgment and decision making of children at preschool and elementary school age and adult controls (N > 1200). Participants’ judgments and decisions were observed in a complex but child-friendly probabilistic inference paradigm, where available cues predict decision outcomes probabilistically. Manipulating the probability distribution and decision feedback created different decision environments, in which children’s utilization of probabilities for judgement and decisions was assessed. Results consistently show a stable developmental pattern: Children aged six do not utilize probability in decision making, children aged nine partly do. Probability utilization in judgment follows the same developmental trajectory: At age six, children seldom rely on probability when making judgments, at age nine, children consider probability, but only a minority does so as consistently as adults do. However, even 6-year-olds do not decide randomly. Instead, children mostly rely on non-adaptive decision strategies that neglect probability but consider irrelevant information in a very systematic fashion. 9-year-olds still use non-adaptive decision strategies, but less often, and start to apply adaptive, probability-based decision strategies. Decision feedback—immediately experiencing decision outcomes—can improve children’s decision quality. It reduces non-adaptive decision strategies and promotes adaptive strategies when it is provided ideally, that is, when feedback either explicitly reveals that decision strategies are non-adaptive or strongly promotes simple, adaptive decision strategies. Altogether, this research shows that already at age six, children possess some competences crucial for good decision making. They can, for example, apply decision strategies very consistently and learn form decision feedback. However, deficits in probabilistic judgment and decision making persist until the age of nine, when normative utilization of probability only starts to emerge.